Expression of chicken fetal antigens (CFA), thirteen serologically distinct oncodevelopmental membrane antigens, will be analyzed in embryonic development and in cellular differentiation of erythroid and myeloid cells in normal and cancerous chickens, as well as in in vitro cell lines. The possibility that CFA is an important membrane receptor for cell-cell communication in embryonic development will be investigated by analyzing CFA expression on the primitive erythroid series in normal 12 hour-11 day embryos and screening for CFA abnormalities in previously identified developmental mutants. The possibility that CFA determinant expression may be associated with specific cellular differentiation-cell cycle phases will be analyzed in normal and erythroleukemic chickens, and virally transformed lymphoid cell lines. Breeding experiments will be performed in order to determine the inheritance pattern of CFA determinants. The role of genetic versus environmental factors in CFA expression will be investigated in in vitro analyses of erythroid cellular maturation of cells obtained from normal embryos and adult bone marrow. The possibility that CFA may serve as a viral receptor will be further analyzed. The relationship of AFP and suppressor T lymphocytes, both expressed during embryonic development, to suppression of the immune response system and tumor induction will be investigated.